354 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



osmotactic reactions of these organisms are of phobic or tropic nature. 

 The bacteria hitherto examined are stimulated by the same substances 

 as the Flagellatae mentioned, but carry out phobic reactions alone. The 

 curvatures of fungal hyphae have, however, a normal tropic character, 

 and the same appears to apply to the positively chemotactic movements 

 of Plasmodia, although the precise nature of their negatively chemotactic 

 and osmotactic movements is doubtful. 



It has been seen that it is often doubtful whether a substance exercises 

 one or more stimulatory actions, and still less is known of the mode of percep- 

 tion and the conditions for it. The osmotropic and hydrotropic irritabilities 

 might possibly be based upon similar sensibilities, although in some cases 

 at least this does not appear to be the case. A few facts are, however, 

 known concerning the minimal stimuli for response, the effect of the 

 intensity of the stimulus, and the power of discrimination. 



In the case of sensitive organisms a very small amount of a good 

 stimulatory material suffices to produce a perceptible reaction. Anthero- 

 zoids and Bacteria are attracted to a capillary tube containing a hundred- 

 millionth of a milligram (0-00,000,000,001 gram) of malic acid or of 

 peptone 1 respectively, and this although only a small fraction actually 

 comes into contact with each excitable organism. These quantities are, 

 however, not as small relatively as they appear, since an antherozoid 

 is about o-ooo,ooo,ooo,o2'5, and a Bacterium ttrmo 0-000,000,000,002 of 

 a gram., i.e. the material in the tube weighs five times as much as the 

 Bacterium termo, and has ^ the weight of the antherozoid. Negative 

 osmotaxis, on the other hand, is only exhibited in the presence of solutions 

 whose concentrations are equivalent to at least 0-5 per cent, potassium 

 nitrate solution. 



It appears that the chemotactic and osmotactic sensitivities of certain 

 micro-organisms are extremely changeable. Thus Massart 2 found that 

 Spirillum undula after gradual accommodation to saline solutions required 

 a salt solution of from five to eight times the previous concentration to 

 produce perceptible repulsion. 



The sensitivity may be lowered by unfavourable conditions, and 

 Voegler 3 has shown that at low temperatures the antherozoids of Ferns 

 require stronger solutions to produce a perceptible reaction than they do 

 at ordinary temperatures. It remains, however, to be seen whether the 

 sensitivity is lost sooner at low temperatures or in the absence of oxygen 



1 Pfeffer, Unters. u. d. bot. Inst. zu Tubingen, 1884, p. 382 ; ibid., 1888, p. 628. [A trace of 

 oxygen may suffice for the movement of aerobic bacteria without being able to produce any perceptible 

 aerotaxis. This is well shown when the bacterium method is used to detect photosynthesis in isolated 

 chloroplastids.] 



2 Massart, 1. c., 1889, p. 548. 



8 Voegler, Bot. Ztg., 1891, p. 673. Cf. also Stange, Bot. Ztg., 1890, p. 139, in regard to the 

 zoospores of Saprolegnia. 



